Global navigation satellite systems GPS and GLONASS, now being deployed in the United States and Russia, represent a revolutionary change in navigation and positioning technology. Other such systems will likely follow, which will be applicable to all forms of travel. One of the areas affected profoundly by the availability of satellite based navigation (SatNav) is civil aviation, where these systems show an enormous promise for enhancing economy as well as safety. Airports of the future will likely be equipped with local-area differential GPS, as well as wide-area differential GPS, particularly Wide Area Augmentations System (WAAS). Planning is underway in the U.S. to switch to GPS-based navigation and surveillance in all phases of flight including precision approaches.
Precision approaches, carried out under poor visibility conditions, require navigational guidance both horizontally and vertically, and place stringent requirements on the accuracy of position estimates. Vertical guidance, in particular is of critical importance. Currently, precision approaches require local instrumentation, such as Instrument Landing System (ILS), and the Microwave Landing System (MLS), installed at each runway to provide navigational guidance to suitably equipped aircraft. The ILS and MLS are expected to be phased out in favor of satellite navigation. In the future, precision approaches using satellite navigation will use GPS in either differential mode, local mode, or wide area mode. The latter of such may be Wide Area Augmentation System (WAAS), which is planned to provide such capability over conterminous U.S. by the year 2000. Satellite navigation systems however, are thought to offer significantly better horizontal than vertical position estimates. Thus, an important challenge in the industry is to provide systems with enhanced vertical position accuracy.
Precision approaches under a Category I standard used internationally require horizontal and vertical navigational guidance down to an altitude (or "decision height") of 200 feet above the touchdown area. If the runway is in view when the plane reaches a decision height the pilot may land using the visual references for navigation. The decision heights for Category II and Category III standards are significantly lower.
The deviation from a pre-defined flight path allowed an aircraft while being guided by a navigation system for a precision approach is strictly limited. As expected, the permissible deviation, called an alarm limit, changes with aircraft altitude. Thus, the closer the aircraft gets to touchdown, the less deviation allowed. Alarm limits are specified for both horizontal and vertical errors at each altitude. If at any point during an approach the aircraft deviation from the prescribed flight path exceeds the alarm limit, the pilot is warned promptly and the approach is aborted. Insofar as the aircraft position can only be estimated using satellite navigation, there will be some uncertainty associated with it. In order to ensure that the aircraft doesn't violate an alarm limit, the navigation system must be required to deliver position estimates of assured accuracy. The lower the aircraft altitude, the lower the deviation alarm limit, and the lower the error tolerable in the position estimate. If a position estimate cannot be assured by the navigation system to meet the current requirement on its accuracy, the pilot is warned. This function of ensuring that the quality of a position estimate is acceptable is referred to as integrity monitoring.
Conventional methods of integrity monitoring have operated under the premise that, if assured of system integrity each user could count on obtaining a position estimate of a certain quality. This premise, however, does not hold for satellite navigation in general. For example, different users of a satellite navigation system may obtain position estimates of significantly different qualities. Thus, in adopting satellite navigation for civil aviation, an important issue to be resolved is providing the user (e.g., the pilot of an aircraft in this scenario) with a system capable of recognizing if a position estimate is good enough for a precision approach. This is even more important in the industry considering that conventional receivers often provide poor vertical position estimates.
Generally, the quality of position estimates obtained by different users of a satellite navigation system vary greatly. Conventional receivers operating with satellite navigation systems measure the transit time of the signal and decipher the data to determine the satellite position. Given that the distance from a satellite to the receiver is determined by the speed of light multiplied by the signal travel time, it is important that time measurement be characterized accurately. For example, if the satellite clock and the receiver clock were out of sync by even 0.001 second, the measurement of distance from the satellite to the receiver would be off by 1,860 miles. If receiver clocks were perfectly synchronized with the satellite clocks, only three measurements (x,y,z) of range to satellites would be needed to allow a user to compute a three-dimensional position. This process is known as mulitlateration. However, given the expense in providing a receiver clock whose time is exactly synchronized, a way to account for receiver clock bias has been to compute a measurement from a fourth satellite. This is done by a processor in the receiver which correlates the ranges measured from each satellite to where they intersect. If a series of measurements do not intersect, the processor either subtracts or adds time from all of the measurements, continuing to do so until it reaches a three-dimensional position estimate where all range estimates intersect. This is carried out by the use of basic trigonometry, usually four equations with four unknowns x,y,z,b. The amount b, by which the processor has added or subtracted time to achieve this intersection, is the bias between the receiver clock and the satellite clock.
Having measurements obtained from four satellites in view, however, does not assure a good position estimate. The biased range measurements are called pseudoranges. Estimation of the four unknowns, x, y, z, b where b is the clock bias estimate, are referred to as 4-D estimations. The quality of a position estimate depends upon two factors: (1) the number of satellites in view and their spatial distribution relative to the user, and (2) the quality of the "pseudorange measurements". Satellite geometry is characterized by a parameter called "Dilution of Precision" (DOP). This parameter, DOP, can be thought of as roughly inversely proportional to the volume of a polyhedron with the receiver being at the apex and the satellite positions defining the base. The pseudorange measurements are range measurements which contain errors, thus the quality of pseudorange measurements is characterized by their rms error. There are several sources of error which affect range measurements including errors in the predicted ephemeris of the satellites, instabilities in the satellite and receiver clocks, ionospheric and tropospheric propagation delays, multipath, and receiver noise. Further errors in position estimates may be the result of the effects of an undetected or an unannounced system malfunction. The collective effect of these errors is referred to as the User Range Error (URE) and its rms value is .alpha.URE. The position error is thus expressed in terms of these two factors: RMS position error=(DOP)(.alpha.URE). In order for satellite navigation to be used globally, all users must have in view at least four satellites geometrically positioned for accuracy as well as a URE such that the resulting position estimate meets the user's requirement. As stated above, Category I approaches determine a range within which an aircraft must stay when performing a precision approach.
Known systems generally cannot provide each user with the ability to accurately determine on the basis of satellite measurements when a position estimate meets accuracy requirements, and when it does not. Characterization of the accuracy of a position estimate in terms of the measurements themselves is referred to as receiver autonomous integrity monitoring. Currently, however, methods of integrity monitoring rely on inaccurate vertical position estimates obtained by conventional receivers. Some work in the area of developing methods of receiver autonomous integrity monitoring has dealt with guarding against measurement anomalies due to satellite malfunctions only, which is a factor responsible for introducing large errors in position estimates. Unfortunately, these methods require redundant measurements and can deal effectively only with one anomalous measurement. If multiple anomalies arose, these methods of integrity monitoring would fail to detect the problem and provide such detection to the user.
The instant invention overcomes the problems of obtaining accurate vertical position estimates, and providing the user with an indication as to the accuracy of the vertical position measurement for an intended purpose, notwithstanding the degree of error or the number of anomalies effecting the error.